1. Field of the Invention
The present invention relates to improvements in a process for the preparation of p-aminophenol from nitrobenzene.
2. Description of the Background Art
The compound p-aminophenol (PAP) is an important chemical intermediate used in the preparation of the analgesic acetaminophen. A number of other derivatives having a wide variety of industrial applications may also be prepared from PAP. One method for preparing PAP involves the catalytic hydrogenation of nitrobenzene in an acid medium. In this process, phenylhydroxylamine is first formed, and this intermediate promptly rearranges in the acid reaction medium to form PAP.
Henke, et al. (U.S. Pat. No. 2,198,249) reportedly were the first to disclose a process for the preparation of PAP by the catalytic hydrogenation of nitrobenzene in an acid medium.
A number of process variations have since been disclosed. For example, Spiegler (U.S. Pat. No. 2,765,342) studied the reaction extensively. Among the factors explored by Spiegler was the effect on reduction rate and p-aminophenol yield of including a surfactant selected from among quaternary ammonium compounds and several non-quaternary compounds. Among the quaternary ammonium compounds used was dodecyl trimethylammonium chloride. The non-quaternary compounds investigated include two simple tertiary amine salts: triethylamine sulfate and tributylamine sulfate, as well as dioctadecyl propyleneamine dioleate. From a plot of rate and yield data, Spiegler concluded that the rate/yield performance of all of the quaternary ammonium compounds examined was superior to that of the non-quaternary compounds.
Later, Brown et al. (U.S. Pat. No. 3,535,382) reported that certain nonionic polyether polyol surfactants could be substituted for Spiegler's quaternary ammonium compounds.
R.G. Benner (U.S. Pat. No. 3,383,416) used the Henke et al. approach of charging all the nitrobenzene at once, but used a carbon-supported platinum catalyst and quaternary ammonium surfactant, preferably dodecyl trimethylammonium chloride, as disclosed by Spiegler. Benner purposely interrupted the hydrogenation well before all the nitrobenzene had been consumed. In the presence of two liquid phases, aqueous and nitrobenzene, a carbon-supported platinum catalyst is preferentially wetted by the nitrobenzene, so most of the catalyst is suspended in the nitrobenzene layer, permitting the removal of the upper aqueous solution of PAP and aniline by decantation. The PAP is then recovered from the aqueous layer and purified.
While Benner proposed quaternary ammonium salts as the surfactant, other surfactants have been utilized, including amine oxide (Sathe, U.S. Pat. No. 4,176,138) and amine salts (Derrenbacher, U.S. Pat. No. 4,307,249). Use of these surfactants tends to result in formation of stable emulsions, which makes separation of the desired PAP product difficult.
There remains a need in the art for improvements in the process for preparing p-aminophenol by catalytic hydrogenation of nitrobenzene.